Mail from outside the AOL system to AOL users was apparently re-routed to other destinations, possibly due to a hacker's attack that resulted in a key DNS address being changed, said AOL spokeswoman Ann Brackbill.

"We have no idea who was behind it at this point, but we are making every effort to find out what happened," Brackbill said. The hack -- which also left an undisclosed number of users with "bring your own access" plans unable to get onto AOL for several hours Friday -- was discovered when systems administrators began noticing that mail to AOL was bouncing back, she said.

The problem has been fixed, and mail should now be getting through normally, Brackbill said Friday afternoon.

Only a small percentage of users are on the "bring your own access" plan, she said, adding that it was "hard to tell how many people were affected" by the outage.

But about half the e-mail sent to AOL's 13 million members each day comes from outside AOL, she said.